The present invention pertains to a method for powder coating plastic parts or articles and parts or articles made by the method. More particularly, the present invention pertains to a method for powder coating plastic, non-conductive articles using resin-type coating materials, without the need for electrically grounding the articles, and articles made by this method.
Myriad articles are coated in order to protect the articles and to provide an aesthetically appealing appearance. For example, many automobile parts are coated, e.g., chromed, to provide a desired aesthetic effect. Often, metal parts are chrome plated using well-known, liquid chrome-plating techniques. However, one drawback to this plating method is that undesirable chemical compounds are a byproduct of the process. These compounds must be contained and treated to avoid environmental concerns.
In another known coating method, a metal part is grounded and a coating is applied to the part, as a powder, through an electrostatic spray device, e.g., gun. The powdered coating is attracted to the part by the opposite charges of the coating particles and the article to be coated. In such a process, it is necessary to maintain the articles grounded in order to effect the electrostatic attraction between the coating particles and the article.
It is also known to coat articles with liquid paint. For example, a paint may be applied to any type of article (metallic or non-metallic) by known methods. In such painting techniques, the paints are carried in a vehicle that typically contains high levels of volatile organic compounds (VOCs). In the painting and curing stages, these VOCs are emitted into the work environment and possibly into the atmosphere. As will be appreciated, VOCs are environmentally undesirable, often ozone depleting compounds. As such, the emission of VOCs is an undesired side effect of conventional painting techniques.
In that plastic parts typically cannot be electrically grounded, one known method for powder coating plastics requires that the plastic article be coated with a conductive material prior to the application of the powder. That is, it has been found that in order to properly coat the plastic parts, it is necessary to pre-treat the parts with, for example, a solution of a salt and isopropyl alcohol. Upon drying of the alcohol, the salt remains on the surface of the plastic article and thus provides the necessary conductivity to electrically ground the article. This, however, has been found to be an expensive, time consuming and inefficient method for coating such plastic articles.
It has also been found that some of the known techniques for applying a coating (e.g., painting or powder coating) to plastic articles do not provide an acceptable quality level of the coating on the part. One known criteria is the visual smoothness of part coating, referred to as an orange peel rating. On a relative scale, glass has a 10 rating and an orange peel has a 1 rating. An acceptable level for coated part smoothness for certain industries is a rating of 7 or greater. Various automobile manufacturers have internal quality procedures and standards for determining orange peel ratings.
Accordingly, there exists a need for a method for powder coating plastic parts or articles. Desirably, such a method is an efficient and cost-effective method for coating plastic articles using resin-type powder coating materials. More desirably, such a method can be used with non-conductive plastic articles. Most desirably, such a method reduces the overall emission of VOCs while providing a high quality, protective and aesthetically appealing powder coating on such articles. Further still, a desirable method provides powder coated articles having an acceptable cosmetic or aesthetic appearance with increased resistance to environmental conditions.